The concept of “digital cinema” includes the production, delivery and presentation of aural/visual material in auditoriums or theatres using digital technology. Digital cinema programs typically are distributed in a compressed and encrypted form on physical media such as DVD-ROM, tape or computer hard drives and can in principle be distributed by electronic transmission using satellite or other broadband communication paths.
Digital cinema playback systems control the processes required to make a digital cinematic presentation. These processes include receiving and storing the digital cinema program, decompressing and deciphering it into digital video and audio data streams that can be processed by digital content decoders, decoding the content of the data streams to obtain signals that may be used drive video displays and audio amplifiers, and controlling other facilities such as curtains or theatre lighting that are found in a theatre auditorium.
For a variety of business reasons it is necessary to convey and process video and audio content in separate data streams. For example, video data for one presentation may be distributed with multiple sets of audio data having different languages or differences in vulgarity of speech that affect the maturity rating of the presentation. At playback time, the appropriate audio data can be selected for presentation with the video data. The video data selected audio data are conveyed in independent streams to equipment for processing such as content decoding.
Typical digital cinema playback systems include several pieces of equipment that communicate with one another through an electrical network that is similar to many networks that are used to interconnect computers. These networks often conform to a standard that is commonly known as Ethernet, which is described in the IEEE 802.3 standard, using a communication protocol known as the Transmission Control Protocol/Internet Protocol (TCP/IP). This choice of network and protocol can simplify the task of implementing a digital cinema playback system because the electrical and logical interfaces and procedures needed to use them are readily available and have relatively low cost. Unfortunately, the use of this type of network and protocol makes it difficult to synchronize the processing and presentation of the video and audio data streams.
A lack of synchronization between the video and audio data streams can introduce differences in the timing of events in the video and audio content that are intended to occur simultaneously. If the difference in timing is great enough, the result can be distracting to a viewer. Synchronization errors may be caused by timing errors in a playback systems or they may be caused by errors in control information conveyed within the video and audio data streams that control the operation of the playback systems. Control information errors may arise from a variety of sources including mistakes made during authoring or mastering of the video/audio content, conversions of the data streams from one data format or standard to another, and a difference in the lengths of the video and audio content, which introduces alignment errors when a presentation is switched from one program to another.
There are known methods for synchronizing audio and video data streams in a variety of contexts including the synchronization of streams that are conveyed in packets over networks such as IP networks. Many of these methods have been designed for use where network delays are unpredictable, packets of audio or video content are lost or received with errors, and competing network traffic is variable. Common approaches that are used to maintain synchronization between independent video and audio data streams include dropping or discarding packets of information from whichever data stream is behind the other, adjusting the speed of the processing clock for equipment that processes the data streams, and truncating processes that decode or recover data from received packets.
Unlike other applications for digital video and audio content, a digital cinema playback system should not drop or discard packets of video and audio content and the quality of the recovered video and audio content must kept at its highest level. Adjustment to the speed of processing clocks is either not possible in some implementations or is unattractive because it can introduce noticeable distortion in the presentations.